Screech in rocket nozzles undergoing free shock separation

Flow separation in rocket nozzles generates intense side loads that can lead to structural damage affecting space flight safety. Recent studies have hypothesised that a screech-like resonance loop may be responsible for such oscillations in the free shock separation (FSS) regime. In this work, we aim to confirm if screech is indeed present in this flow. Five thrust-optimised parabolic nozzles with different length and area ratios were examined at a nozzle pressure ratio range where FSS was expected to occur. Spectral proper orthogonal decomposition (SPOD) revealed resonance for both m equals 0 m = 0 $m=0$ and m equals 1 m = 1 $\,m=1$ azimuthal modes. Further decomposition of the SPOD modes allowed for the identification of downstream- and upstream-travelling waves, whose characteristics match the description of the Kelvin–Helmholtz (KH) instability wave and the guided jet mode (GJM), respectively. Linear stability analysis performed on Reynolds-averaged Navier–Stokes mean-flow fields confirms the existence of these two waves for the conditions studied and provides insight into their dispersion relation. A well-validated screech predictive model was employed and accurately predicted the resonant frequencies. The accurate predictions, combined with the observation of the KH and GJM waves, confirm that screech is active in this flow.